3-46approaches an infinite value, E_{R} increases and approaches E_{i} in value. When R_{L} is smaller than Z_{0}, E_{R} hasa negative value. This means that the reflected voltage is of opposite polarity to the incident wave at thetermination of the line. As R_{L} approaches zero, E_{R} approaches E_{i} in value. The smaller the value of E_{R}, thesmaller is the peak amplitude of the standing waves and the higher are the minimum values.TERMINATION IN A RESISTANCE GREATER THAN Z_{0}When R_{L} is greater than Z_{0}, the end of the line is somewhat like an open circuit; that is, standingwaves appear on the line. The voltage maximum appears at the end of the line and also at half-waveintervals back from the end. The current is minimum (not zero) at the end of the line and maximum at theodd quarter-wave points. Since part of the power in the incident wave is consumed by the load resistance,the minimum voltage and current are less than for the standing waves on an open-ended line. Figure 3-34,view G, illustrates the standing waves for this condition.TERMINATION IN A RESISTANCE LESS THAN Z_{0}When R_{L} is less than Z_{0}, the termination appears as a short circuit. The standing waves are shown infigure 3-34, view H. Notice that the line terminates in a current LOOP (peak) and a voltage NODE(minimum). The values of the maximum and minimum voltage and current approach those for a shortedline as the value of R_{L} approaches zero.A line does not have to be any particular length to produce standing waves; however, it cannot be aninfinite line. Voltage and current must be reflected to produce standing waves. For reflection to occur, aline must not be terminated in its characteristic impedance. Reflection occurs on lines terminated inopens, shorts, capacitances, and inductances, because no energy is absorbed by the load. If the line isterminated in a resistance not equal to the characteristic impedance of the line, some energy will beabsorbed and the rest will be reflected.The voltage and current relationships for open-ended and shorted lines are opposite to each other, asshown in figure 3-34, views C and D. The points of maximum and minimum voltage and current aredetermined from the output end of the line, because reflection always begins at that end.Q26.A nonresonant line is a line that has no standing waves of current and voltage on it and isconsidered to be flat. Why is this true?Q27.On an open line, the voltage and impedance are maximum at what points on the line?STANDING-WAVE RATIOThe measurement of standing waves on a transmission line yields information about equipmentoperating conditions. Maximum power is absorbed by the load when Z_{L} = Z_{0}. If a line has no standingwaves, the termination for that line is correct and maximum power transfer takes place.You have probably noticed that the variation of standing waves shows how near the rf line is tobeing terminated in Z_{0}. A wide variation in voltage along the length means a termination far from Z_{0}. Asmall variation means termination near Z_{0}. Therefore, the ratio of the maximum to the minimum is ameasure of the perfection of the termination of a line. This ratio is called the STANDING-WAVE RATIO(swr) and is always expressed in whole numbers. For example, a ratio of 1:1 describes a line terminated inits characteristic impedance (Z_{0}).